Modern spark-ignition internal combustion engines are generally equipped with sensors and associated control systems operable to detect auto-ignition of a combustion charge, also referred to as detonation or engine knock. The auto-ignition of an air/fuel combustion charge generally occurs under severe engine operation, including high load operation or high temperature operation, especially in engines operating at high compression ratios. Under such conditions, a combination of thermal energy transferred to the combustion charge, coupled with increase in combustion pressure due to engine load, causes uncontrolled, rapid, and uneven combustion of the air/fuel charge in the cylinder. As such, knock occurs when portions of the air/fuel combustion charge in the cylinder auto-ignite, instead of a controlled ignition initiated by an electrical arc created in a spark plug inserted into the combustion chamber. A auto-ignition event creates unplanned and uncoordinated pressure pulses in the combustion chamber that induce thermal and vibrational stresses on various engine components, including pistons, crankshaft, and intake and exhaust valves. The induced thermal and vibrational stresses may lead to catastrophic failures of the engine components.
Manufacturers of modern engines and control systems have implemented systems to control and compensate for engine knock. The control systems include sensors and control systems operable to detect auto-ignition of a combustion charge which change spark timing of the engine to reduce the occurrence of knock events. Such control systems employ one or more knock sensors attached to the engine near the cylinders. Knock sensors are generally piezoelectric accelerometer devices operable to sense vibration. Output from each knock sensor is input to the engine controller. When a filtered signal from the sensor exceeds a threshold value, the controller implements control schemes to reduce knock, generally by retarding timing of spark ignition. The ignition retard scheme may be specific to a cylinder, to a bank of cylinders, or globally applied to all cylinders in the engine. A knock control scheme generally continues until the knock signal is below the threshold value.
Detection of spark knock in an internal combustion engine is confounded by the presence of signal noise and vibrations incidental to normal combustion and engine operation. Both electrical and mechanical noises and vibrations may be included in a signal from a knock sensor. Mechanical noise, as may be caused by closing of engine valves is especially significant with an accelerometer-type knock sensor, because impact of a valve against a valve seat typically sends a sharp vibration throughout the engine.
Skilled practitioners have typically addressed the issue of noise caused by valve closing by placing the knock sensor in a location that minimizes magnitude of noise introduced by valve closing, but still able to detect engine knock. The problem has also been addressed by employing multiple sensors on an engine, so any significant knock signal in each cylinder is sensed by at least one of the sensors. The engine controller is then programmed to select the appropriate sensor for knock sensing and detection in each cylinder, using predetermined signal sampling techniques and signal-to-noise ratio analysis. Skilled practitioners have also typically addressed the issue of noise caused by valve closing by time windowing, wherein the controller monitors the knock sensor signal only during a specific time window, based upon engine rotational position. The time window is defined to avoid valve closing noise, yet capture a significant portion of the spark knock signal. Most skilled practitioners tasked to implement a knock detection system for a modern engine use a combination of the above two methods. A knock system may be readily implemented to avoid signal noise problems caused by normal combustion operation through the effective design and placement of the knock sensor in conjunction with time windowing strategies for monitoring signal output.
Implementation of variable valve control systems onto modern engines may affect the ability of the controller to accurately discriminate between normal engine noise and spark knock. A typical variable valve control system comprises a cam phasing system that adjusts opening and/or closing times of engine valves relative to piston position to improve or alter engine operating characteristics. Any change in timing and magnitude of normal engine mechanical noise due to an adjustment of opening and/or closing time of engine valves must be filtered to permit accurate sensing of engine knock. The knock control scheme is typically able to accommodate adjustment of opening and/or closing time of engine valves by adjusting the time window during when the knock sensor signal is monitored. This scheme works when the adjustment in cam phasing is predictable.
Under certain conditions, actual cam position may deviate from cam position scheduled by the controller. Such conditions occur when cam phasing is disabled due to excessive engine oil temperature or other engine control or protection schemes. Actual cam position may deviate from scheduled cam position during transient events, i.e. when the cam phaser is changing rotational position of the cam relative to the crankshaft. Under these conditions the actual cam position does not match the scheduled cam position due to response lag of the cam phaser. This may lead to an inability to filter actual engine and valve noise resulting in a false detection of engine knock and inappropriate spark correction to reduce knock. This may instead lead to an inability to detect an actual spark knock event. In both circumstances, there is a possibility of reduced engine performance in terms of power, fuel economy, or audible knock noise. There is risk of damage to the engine and catalytic converter due to excess knock.
Engineers have attempted to address the problem by redesigning and implementing improved knock sensing hardware and enhancing signal filtering of knock signals, to improve rejection of engine operation noise, including valve closing noise. Such efforts have not been completely successful. There is therefore an ongoing need to detect auto-ignition events on modern engines equipped with variable valve timing systems, including variable cam phasing systems.